1. Field of the Invention
The present invention relates to an image processor for use in digital color image forming apparatuses such as a digital color copier and more specifically to a gradation correcting process.
2. Description of Prior Art
There is disclosed a prior art related to a gradation correction process in Japanese Unexamined Patent Publication JP-A 4-180354 (1992). A digital copier provided with a density correcting unit of JP-A 4-180354 comprises an electrophotographic printer. In the printer, a photosensitive film is exposed to a modulated light beam which is controlled by an optically modulated signal. The density correcting unit carries out a gradation correcting process while converting an image signal of an object to be processed into the optically modulated signal. To that end, the density correcting unit stores preset desired characteristics of the digital copier and a preset reference conversion table. The desired characteristic indicates changes of density of recording density to a density value of an image signal. The reference conversion table is a reference correction curve indicative of predetermined changes of a value of the optically modulated signal with respect to the density value within the image signal.
The density correcting unit carries out the following procedure to create a signal conversion table which is a correction curve actually used in processing the image signal. The density correcting unit generates a plurality of reference image signals whose density values are different from each other, converts the respective reference image signals into the optically modulated signals, respectively, based on the reference conversion table and supplies them to the printer. The printer prints a recorded image corresponding to the respective reference image signals by modulating the light beams based on the respective optically modulated signals, by exposing the photosensitive film by using the light beam and by developing the photosensitive film after the exposure. The density correcting unit measures the density of the respective printed recorded images and finds a density value of the desired characteristics to each density.
Next, the density correcting unit calculates a deviation between a value of the optically modulated signal on the reference conversion table to the density value of the desired characteristics to the density of the respective recorded images and a value of the optically modulated signal on the reference conversion table to the density value of the reference image signal corresponding to the respective recorded images. Then, the density correcting unit carries out an interpolation process using the deviation of the density values of the reference image signal to obtain a deviation of a density value other than the density value of the reference image signal to set the deviation of the density value as continuous data. The interpolation process is a publicly known interpolation and is realized by a combination of spline interpolation, linear interpolation and smoothing or by n-order function approximation.
Finally, the density correcting unit corrects the reference conversion table to obtain a value on the signal conversion table. A value of an optically modulated signal on the reference correction curve to the density value of the reference image signal corresponding to the respective recorded images is substituted into the correction value on the signal conversion table with respect to the density value on the desired characteristics to the density of the respective recorded images. A correction value on the signal conversion table with respect to a density value other than the density value on the desired characteristics to the density of the respective recorded images may be obtained by correcting the reference conversion table based on the data of the continuous deviation. The signal conversion table obtained as the result of the above-mentioned process is actually used in the density correcting unit.
The density correcting unit in the above-mentioned JP-A 4-180354 carries out the interpolation in order to obtain the data of deviation of the continuous density values as described above. The interpolation is realized by the combination of spline interpolation, linear interpolation and smoothing or by the n-order function approximation. When these known interpolations are used, no reference is made to the reference conversion table in the interpolation of the deviation. Thus, the output characteristics of the signal conversion table is liable to lack the reliability.
It is an object of the invention to provide an image processor capable of improving the reliability of the output characteristics of the correction curve actually used in the gradation correcting process.
In a first aspect of the invention, an image processor comprises image inputting means for inputting an image, gradation correction processing means for performing a gradation correcting process using a correction curve to the inputted image and image outputting means for outputting the image for which the gradation correcting process has been performed, the image inputting means supplying input density values of pixels composing the image, to the gradation correction processing means, the gradation correction processing means supplying correction values on the correction curve to the input density values, to the image outputting means, the image processor further comprising:
reference correction curve storing means for storing a reference correction curve indicative of predetermined changes of the correction values to the input density values; and
correction amount storing means for storing correction amounts wi and wi+1 which are preset based on the reference correction curve and correspond to a plurality of specified density values XPi and XPi+1, respectively, which are input density values specified at intervals from each other within the range of the input density values,
wherein the correction amount wi corresponding to the specified density value XPi is a difference between a correction value YB(XPi) on the reference correction curve to the specified density value XPi and a correction value YP(XPi) to the specified density value on the correction curve,
the gradation correction processing means, in order to create the correction curve to be used for the gradation correcting process,
(a) calculates the correction values YP(XPi) and YP(XPi+1) to the specified density values XPi and XPi+1 on the correction curve, by adding the correction values to the specified density values XPi and XPi+1 on the reference correction curve and the correction amounts wi and wi+1 corresponding to the specified density values XPi and XPi+1, and
(b) calculates the correction value to the input density value other than the specified density value by means of interpolation operation for causing a shape of a part of the correction curve within a first section xcex94XPi having the specified density values XPi and XPi+1 as both ends thereof, to coincide with a shape obtained by expanding or compressing in the direction X parallel to the axis of density value, a part of the reference correction curve within a second section xcex94YBi having the correction values YP(XPi) and YP(XPi+1) to the specified density values as both ends thereof.
According to the first aspect of the invention, the gradation correction processing means in the image processor can create the correction curve smoother than those created by using an interpolation operation linear interpolation quickly by easier calculation than the case of using spline interpolation or n-order function approximation. The output characteristics of the correction curve per first section having the both ends of the specified density values is analogous to the output characteristics of the reference correction curve in the second section having the both ends of the correction values to the same specified density values on the correction curve as those of the both ends of the first section. Thereby, an output section whose output characteristics based on the reference correction curve is almost linear has the almost linear output characteristics even after when a correction is made based on the created correction curve.
According to the first aspect of the invention, the gradation correction processing means in the image processor finds a part of the correction curve by means of interpolation operation of expanding or compressing a part of the reference correction curve in the direction parallel to the axis of the density value by moving it in that direction when creating the correction curve by using the reference correction curve and the correction amount to the specified density values. It allows a smooth correction curve to be obtained quickly by simpler calculation than the case of using the spline interpolation or the n-order function approximation for the interpolation operation. Still more, because the output characteristics of the correction curve is analogous with the output characteristics of the reference correction curve per each section, the reliability of the correction curve may be improved.
In a second aspect of the invention, the image processor is characterized in that during the interpolation operation, the gradation correction processing means:
(a) calculates a third section xcex94XBi which is a section having input density values XB(YBi) and XB(YBi+1) on the reference correction curve to the correction values YP(XPi) and YP(XPi+1)to the specified density values, as both ends thereof;
(b) correlates the input density value Ji within the third section with the input density value XX(Ji) within the first section so that the both ends of the third section xcex94XBi coincide with the both ends of the first section xcex94XPi; and
(c) uses the correction value YB(Ji) to the input density value Ji within the third section on the reference correction curve as the correction value YP(XX(Ji)) on the correction curve to the input density value XX(Ji) within the first section correlated with the input density value Ji.
According to the second aspect of the invention, the gradation correction processing means in the image processor can calculate the correction value to the input density value within the first section on the correction curve, based on the input density value within the third section and the correction value to the input density value on the correction curve.
In a third aspect of the invention, the image processor is characterized in that in determining input value density XX (Ji) within the first section correlated with the input density value Ji within the third section, the gradation correction processing means:
(a) calculates a value obtained by adding a specified density value XPi at one end of the first section to a product of a ratio of a width XPi+1xe2x88x92XPi of the first section to a width XB(YBi+1)xe2x88x92XB(YBi) of the third section and a distance Jixe2x88x92XB(YBi) from one end of the third section to the input density value within the third section; and
(b) rounds the calculated sum to a predetermined number of digits and correlates the input density value XX(Ji) within the first section which is equal to the rounded value, to the input density value Jiwithin the third section.
According to the third aspect of the invention, when the number of digits of the input density value is predetermined, the gradation correction processing means in the image processor can calculate the correction values to the input density values within the first section on the correction curve, based on the input density values within the third section and the correction values to the input density values.
In a fourth aspect of the invention, the image processor is characterized in that in carrying out the interpolation operation in which the shape of the correction curve at one part within the first section xcex94XPi assumes a shape in which a part of the reference correction curve within the second section xcex94YBi is extended in the direction X parallel to the axis of density value, the gradation correction processing means finds a correction value on the correction curve to the remaining input density value other than the input density value XX(Ji) correlated with the input density value Jiwithin the third section among all input density values within the first section, by means of interpolation operation.
According to the fourth aspect of the invention, although a part of all input density values within the first section cannot be correlated with the input density values within the third section when the first section is wider than the third section, the gradation correction processing means of the image processor can find the correction value to the input density value within the first section which is not correlated with the input density value within the third section readily by the interpolation operation.
In a fifth aspect of the invention, the image processor is characterized in that in carrying out the interpolation operation in which the shape of the correction curve of a part within the first section xcex94XPi assumes a shape in which a part of the reference correction curve within the second section xcex94YBi is compressed in the direction X parallel to the axis of density value, when a plurality of input density values within the third section are correlated to the single input density value XX(Ji) within the first section, the gradation correction processing means uses a correction value to an input density value calculated at last among the plurality of input density values within the third section on the reference correction curve, as the correction value YP(XX(Ji)) on the correction curve to the input density value XX(Ji) within the first section.
According to the fifth aspect of the invention, the gradation correcting process in the image processor can create the correction curve more quickly when the first section is narrower than the third section.
In a sixth aspect of the invention, the image processor is characterized in that in carrying out the interpolation operation in which the shape of the correction curve of a part within the first section xcex94XPi assumes a shape in which a part of the reference correction curve within the second section xcex94YBi is compressed in the direction X parallel to the axis of density value, when a plurality of input density values within the third section are correlated to the single input density value XX(Ji) within the first section, the gradation correction processing means uses a center value of the correction values on the reference correction curve to plurality of input density values within the third section, as the correction value YP(XX(Ji)) on the correction curve to the input density value XX(Ji) within the first section.
According to the sixth aspect of the invention, the gradation correction processing means in the reference correction curve can create the correction curve more suitable to the image outputting means because the shape of the part of the correction curve within the first section created by the gradation correction processing means hardly gets out of the shape as compared to the shape in which the part of the reference correction curve in the second section is compressed in the horizontal direction when the first section is narrower than the third section.
In a seventh aspect of the invention, the image processor is characterized in that one density value among the plurality of specified density values is a lower limit density value within the range of the input density values.
According to the seventh aspect of the invention, the correction value to the input density value smaller than the specified density value other than the lower limit density value may become a more suitable value in the correction curve created by the gradation correction processing means within the image processor. Thereby, the gradation correction processing means can correct the gradation more adequately.
In an eighth aspect of the invention, the image processor is characterized in that one density value among the plurality of specified density values is an upper limit density value within the range of the input density values.
According to the eighth aspect of the invention, the correction value to the input density value greater than the specified density value other than the upper limit density value may become a more suitable value in the correction curve created by the gradation correction processing means within the image processor. Thereby, the gradation correction processing means can correct the gradation more adequately.
In a ninth aspect of the invention, the image processor is characterized in that in creating the correction curve, the gradation correction processing means always uses a value defined in advance per each input density value as a correction value on the correction curve to each input density value located at least in one of the section from the upper limit density value within the range of the input density values to the first input density value set in advance and the section from the lower limit density value within the range of the input density values to the second input density value set in advance.
According to the ninth aspect of the invention, the image processor can always maintain the output characteristics within the section to the characteristics (saturated density) set in advance when, on the correction curve the correction value to each input density value within the section from the upper limit density value to the first input density value set in advance is always the value set in advance. The image processor can also always maintain the output characteristics within the section to the characteristics (not outputted) set in advance when on the correction curve the correction value to each input density value within the section from the lower limit density value to the second input density value set in advance is always the value set in advance.
In a tenth aspect of the invention, the image processor is characterized in that the gradation correction processing means selects any one reference correction curve among a plurality of reference correction curves corresponding to the output characteristics intrinsic to the image outputting means and uses the selected reference correction curve in creating the correction curve when the reference correction curve storing means stores the plurality of reference correction curves.
According to the tenth aspect of the invention, the reference correction curve selected corresponding to the output characteristics intrinsic to the image outputting means is used in creating the correction curve in the image processor. Thereby, the gradation correction processing means can create the correction curve more suitable to the output characteristics intrinsic to the image outputting means.
In an eleventh aspect of the invention, the image processor is characterized in that the gradation correction processing means:
(a) causes the image outputting means to output the image pattern of the plurality of specified density values without performing the gradation correcting process;
(b) causes the image inputting means to read the density values of the outputted image pattern;
(c) compares the read density values with the reference characteristic curve set in advance and correlated with each of the reference correction curves; and
(d) selects the reference correction curve correlated to the reference characteristic curve closest to the read density values.
According to the eleventh aspect of the invention, the reference correction curve is selected based on the result of the image pattern of the specified density value outputted by the image output unit as it is. Thereby, the gradation correction processing means within the image processor can select the most adequate reference correction curve to the output characteristics intrinsic to the image outputting means.
In a twelfth aspect of the invention, the image processor is characterized in that the correction amount corresponding to the specified density value is a first correction amount which is a correction amount based on the output characteristics intrinsic to the image outputting means.
According to the twelfth aspect of the invention, the gradation correction processing means within the image processor can carry out the adequate xcex3 correction process based on the output characteristics intrinsic to the image outputting means.
In a thirteenth aspect of the invention, the image processor further comprises adjustment amount input means for inputting an adjustment amount set in connection with the gradation correcting process of the image and is characterized in that the correction amount to the specified density value is a second correction amount which is a correction amount based on the inputted adjustment amount.
According to the thirteenth aspect of the invention, the correction amount corresponding to the specified density value is defined based on the adjustment amount in the image processor. When the adjustment amount is set corresponding to the output characteristics intrinsic to the image outputting means, the gradation correction processing means can carry out the adequate xcex3 correction process based on the output characteristics intrinsic to the image outputting means. When the adjustment amount is an adjustment amount related to the process for correcting the density value desired by the user of the image forming apparatus, the gradation correction processing means can execute the process for correcting the density value based on the adjustment amount simultaneously with the xcex3 correction process (the case of correcting the output characteristics intrinsic to the image outputting means without including the correction by the adjustment amount is called as the xcex3 correction process in the present invention)
In a fourteenth aspect of the invention, the image processor further comprises adjustment amount inputting means for inputting an adjustment amount set in connection with the gradation correcting process of the image, wherein the correction amount corresponding to the specified density value is a sum of the first correction amount which is the correction amount based on the output characteristics intrinsic to the image outputting means and the second correction amount which is the correction amount based on the inputted adjustment amount.
According to the fourteenth aspect of the invention, the gradation correction processing means within the image processor can execute the adequate xcex3 correction process based on the output characteristics intrinsic to the image outputting means and can execute the process for correcting the density value based on the adjustment amount.
In a fifteenth aspect of the invention, the image processor is characterized in that the second correction amount is a total sum of correction amounts defined per each adjustment amount when a plurality of adjustment amounts are inputted from the adjustment amount inputting means.
According to the fifteenth aspect of the invention, the gradation correction processing means within the image processor can calculate the second correction amount readily when the correction amounts are inputted in overlap.
In a sixteenth aspect of the invention, the image processor is characterized in that it further comprises target characteristic curve storing means for storing a target characteristic curve indicative of predetermined changes of the density of the pixel of the image outputted by the image outputting means to the input density value;
the gradation correction processing means:
(a) performs the gradation correcting process using the reference correction curve to the image pattern of each of specified density values XPi;
(b) causes the image outputting means to output the image pattern of each specified density value XPi whose gradation has been corrected;
(c) causes the image inputting means to read the density YMi of the image pattern of each outputted specified density value; and
(d) compares the read density YMi with the target characteristic curve and decides the first correction amount based on the comparison result in determining the first correction amount.
According to the sixteenth aspect of the invention, the gradation correction processing means within the image processor can set the most adequate first correction amount to the output characteristics intrinsic to the image outputting means.
In a seventeenth aspect of the invention, the image processor is characterized in that in comparing the density YMi of the image pattern of each specified density value read as described above with the target characteristic curve, the gradation correction processing means:
(a) calculates the input density value XB(YMi) on the target characteristic curve to the density YMi of the image pattern of each specified density value;
(b) uses the correction value YB(XPi) on the reference correction curve to the specified density value XPi, as a value V(YMi) on an imaginary curve to the calculated input density value XB(YMi);
(c) calculates a value V(XPi) on the imaginary curve to each specified density value XPi by means of interpolation operation of causing a shape of the imaginary curve within a fourth section xcex94XB(YMi) having as both ends thereof, input density values XB(YMa) and XB(YMb) which are located at both ends of the specified density value XPi and to which values on an imaginary curve have been calculated, to coincide with a shape obtained by expanding or compressing a fifth section xcex94V(YMa) having as both ends thereof, the values V(YMa) and V(YMb) on the imaginary curve to the input density values of the both ends of the fourth section, in the direction X parallel to the axis of density value; and
(d) uses the difference between the value V(XPi) on the imaginary curve to the calculated specified density value and the correction value YB(XPi) on the reference correction curve to the specified density value, as a first correction amount si corresponding to the specified density value XPi.
According to the seventeenth aspect of the invention, the gradation correction processing means within the image processor calculates the value on the imaginary curve to the specified density value by means of the interpolation operation corresponding to the output characteristics based on the part of the reference correction curve within the first section whose end is the specified density value. The interpolation operation executed in setting the first correction amount is realized by the interpolation operation for extending or compressing the part of the reference correction curve by moving it in the direction parallel to the axis of density value. Thereby, the gradation correction processing means can calculate the more adequate correction amount as the first correction amount based on the output characteristics intrinsic to the image outputting means.
In an eighteenth aspect of the invention, the image processor is characterized in that when the read density of the image pattern of the specified density value is out of the preset range of the density where the image inputting means is liable to erroneously read, the gradation correction processing means carries out calculation of values on the imaginary curve using the density and when the read density of the image pattern of the specified density value is within the range of the density, the gradation correction processing means stops the calculation of values on the imaginary curve using the density.
According to the eighteenth aspect of the invention, the gradation correction processing means within the image processor stops the calculation of values on the imaginary curve using the density when the density which is difficult to accurately distinct by the image outputting means is obtained in reading the image pattern. As the density which is difficult to distinct, there may be cited the density which is far higher than the lower limit density and the density which is far lower than the saturation density within the range of the density readable by the image inputting means. The image pattern of the density which is far higher than the lower limit density is a thin color for human eyes. The image pattern of the density which is far lower than the saturation density is a slightly thinner color for human eyes as compared to the saturation density. The image inputting means erroneously reads the density of the image pattern whose density is far higher than the lower limit density as the lower limit density. The image inputting means also erroneously recognizes the density of the image pattern of the density which is far lower than the saturation density as the saturation density. Because the calculation of values on the imaginary curve using the density which is hard to be distinguished is not carried out, the first correction amount corresponding to the specified density value within the low density part and the high density part in the range of the density which can be outputted by the image outputting means becomes more adequate.
In a nineteenth aspect of the invention, the image processor is characterized in that the gradation correction processing means calculates the correction value on the correction curve by using densities YMi and YMi+1 when the difference obtained by subtracting read density YMi of the image pattern of the specified density value XPi from read density YMi+1 of the image pattern of the other specified density value XPi+1 which is greater than the specified density value XPi is a positive value and stops the calculation of values on the imaginary curve using the densities YMi and YMi+1 when the difference is a negative value.
According to the nineteenth aspect of the invention, the gradation correction processing means within the image processor stops the calculation of values of the imaginary curve using the specified density value when the difference between the read densities of the image patterns of the two specified density values is negative. It is because the read density of the image pattern is considered to have become unnatural due to the unstability of the image outputting means or the density measuring error of the image inputting means when the difference is negative. Because the calculation of values on the imaginary curve using the difference is not carried out when the difference is negative, the gradation correction processing means can create a smoother correction curve.
In a twentieth aspect of the invention, the image processor is characterized in that the gradation correction processing means calculates the correction value on the correction curve by using densities YMi and YMi+1 when the difference obtained by subtracting read density YMi of the image pattern of the specified density value XPi from read density YMi+1 of the image pattern of the other specified density value XPi+1 which is greater than the specified density value XPi is a positive value and when the difference is negative, calculates an average density of the density YMi of the specified density value and the density YMi+1 of the other specified density value, calculates an input density value on the target characteristic curve to the average density and uses the correction value on the reference correction curve, corresponding to the average input density value of the specified density value XPi and the other specified density value XPi+1 as a value on the imaginary curve to the calculated input density value.
According to the twentieth aspect of the invention, when the difference between the read densities of image patterns of two specified density values is negative, the gradation correction processing means within the image processor finds values on the imaginary curve by using the average density of the densities and the average input density value of the specified density values. Thereby, gradation correction processing means drum can prevent data to be used in the interpolation operation from decreasing too much in the interpolation operation in calculating the first correction amount.
In a twenty-first aspect of the invention, the image processor is characterized in that when the image is a color image, the reference correction curve and the correction amount corresponding to the specified density value are set per each of a plurality of color components.
According to the twenty-first aspect of the invention, the gradation correction processing means within the image processor can execute the gradation correcting process adequately per each color component.
In a twenty-second aspect of the invention, the image processor is characterized in that it further comprises halftone generating processing means for performing a dither type halftone generating process to the image,
wherein the reference correction curve and the correction amount corresponding to the specified density value are set per size of a dither matrix which can be used by the halftone generating processing means.
According to the twenty-second aspect of the invention, the gradation correction processing means within the image processor can execute the gradation correcting process adequately per size of each dither matrix.
In a twenty-third aspect of the invention, an image processor comprises image inputting means for inputting an image, gradation correction processing means for performing a gradation correcting process using a correction curve to the inputted image and image outputting means for outputting the image for which the gradation correcting process has been performed, the image inputting means supplying input density values of pixels composing the image, to the gradation correction processing means, the gradation correction processing means supplying correction values to the input density values on the correction curve, to the image outputting means, the image processor further comprising:
curve storing means for storing the correction curve,
wherein the correction curve indicates a change of the correction value to the input density value, and is prepared, based on the reference correction curve indicative of a preset change of the correction value to the input density value and a target characteristic curve indicative of a preset change to the input density value of the density of the pixel outputted by the image outputting means, in such a manner that:
(a) an image pattern of each of a plurality of specified density values XPi and XPi+1 which are the input density values specified at intervals from each other within the range of the input density values is subjected to the gradation correcting process using the reference correction curve;
(b) an image pattern of each of the specified density values XPi and XPi+1 whose gradation has been corrected is outputted by the image outputting means;
(c) densities YMi and YMi+1 of the outputted image pattern of each of the specified density values XPi and XPi+1 is read;
(d) input density values XB(YMi) and XB(YMi+1) on the target characteristic curve to the densities YMi and YMi+1 of the image pattern of the respective specified density values are calculated;
(e) correction values YB(XPI) and YB(XPi+1) on the reference correction curve to the specified density values XPi and XPi+1 are adopted as correction values YP(YMi) and YP(YMi+1) on the correction curve to the calculated input density values XB(YMi) and XB(YMi+1); and
(f) a correction value on the correction curve to the input density value within a section xcex94XB(YMi) having the calculated input density values XB(YMi) and XB(YMi+1) as both ends thereof is calculated by an interpolation operation for causing a shape of a part of the correction curve within the section xcex94XB(YMi) to coincide with a shape obtained by extending or compressing a part of the reference correction curve within a section xcex94V(YMi) having as both ends thereof, values V(YMi) and V(YMi+1) on an imaginary curve to the calculated input density values XB(YMi) and XB(YMi+1), in the direction X of the axis of density value.
According to the twenty-third aspect of the invention, in the image processor, the correction curve used by the gradation correction processing means is smoother than the correction curve created by using linear interpolation for the interpolation operation and is as smooth as the correction curve created by using spline interpolation or n-order function approximation for the interpolation operation. The correction curve of the 23-rd aspect of the invention may be created quickly by relatively simpler calculation than the interpolation operation using the spline interpolation or the n-order function approximation. The output characteristics of the correction curve per first section having the both ends of the specified density values is analogous to the output characteristics of the reference correction curve in the second section having the both ends of the correction values on the correction curve to the same specified density values as those of the both ends of the first section. Thereby, an output section where the output characteristics based on the reference correction curve is almost linear has the almost linear output characteristics even after when a correction based on the created correction curve has been made. Further, because the image processor of the twenty-third aspect of the invention creates and stores the correction curve itself in advance, it can execute the gradation correcting process faster than the image processor of the first aspect of the invention by the time of creating the correction curve.